The upcoming standalone LTE in Unlicensed Spectrum (LTE-U) forum and future 3GPP Rel-14 work item on Uplink Licensed-Assisted Access (LAA) intend to allow LTE UEs to transmit on the uplink in the unlicensed 5 GHz or license-shared 3.5 GHz radio spectrum. For the case of standalone LTE-U, the initial random access and subsequent uplink (UL) transmissions take place entirely on the unlicensed spectrum. Regulatory requirements may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so-called listen-before-talk (LBT) procedure should be performed. LBT involves sensing the medium for a pre-defined minimum amount of time and backing off if the channel is busy. Therefore, the initial random access (RA) procedure for standalone LTE-U should involve as few transmissions as possible and also have low latency, such that the number of LBT operations can be minimized and the RA procedure can then be completed as quickly as possible.
Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard, also known under its marketing brand as “Wi-Fi.”
LTE uses orthogonal frequency-division multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT) spread (also referred to as single-carrier FDMA) in the uplink. FIG. 1 illustrates a basic LTE downlink physical resource as a time-frequency grid where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of single carrier-frequency division multiple access (SC-FDMA) symbols in the time domain as OFDM symbols in the downlink.
FIG. 2 illustrates an example LTE time-domain structure. As illustrated, LTE downlink transmissions are organized into radio frames of 10 ms in the time domain, and each radio frame consists of ten equally-sized subframes of length Tsubframe=1 ms. Each subframe comprises two slots of duration 0.5 ms each, and the slot numbering within a frame ranges from 0 to 19. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
In LTE, the Physical Random Access Channel (PRACH) is used for initial network access, but the PRACH cannot carry any user data, which is exclusively sent on the Physical Uplink Shared Channel (PUSCH). Instead, the LTE PRACH is used to achieve uplink time synchronization for a user equipment (UE) which either has not yet acquired, or has lost, its uplink synchronization.
FIG. 3 illustrates an RA preamble format. Specifically, FIG. 3 illustrates the structure of a RA preamble sent on the PRACH where a cyclic prefix (CP) is followed by a preamble sequence derived from a Zadoff-Chu root sequence. In the time domain, the PRACH may span between one to three subframes for frequency-division duplexing (FDD) LTE. Any unused portion of the last PRACH subframe is utilized as a guard period. In the frequency domain, the PRACH spans six resource blocks (1.08 MHz).
Downlink and uplink LTE transmissions are dynamically scheduled. For example, in each subframe, the base station transmits control information on the DL about which terminals can transmit in upcoming UL subframes, and in which resource blocks the data is to be transmitted. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of the control information.
FIG. 4 illustrates a typical downlink subframe. As depicted, the downlink subframe has three OFDM symbols as control. The reference symbols are the cell specific reference symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
According to Rel-8 to Rel-10, only the Physical Downlink Control Channel (PDCCH) is available for carrying RA responses from the eNodeB (eNB) when responding to initial RA transmissions on the UL. However, from LTE Rel-11 and thereafter, resource assignments and RA responses can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH).
A typical LTE contention-based RA procedure for initial network access on licensed carriers consists of four steps. In a first step, the UE, which may include a user equipment or any other wireless device, selects and transmits one out of 64 available PRACH sequences. The transmission location may be based on the PRACH configuration broadcast in the cell system information. This preamble transmission on the uplink (UL) may be known as message1 or msg1.
In a second step, the RA response is sent by the eNB on the downlink (DL). This message may be known as message2 or msg2. Specifically, it may be sent on the PDSCH and indicated using the PDCCH, and addressed with an ID. The Random Access Radio Network Temporary Identifier (RA-RNTI) may identify the time-frequency slot in which the preamble was detected. The RA response conveys the identity of the detected preamble, a timing alignment instruction to synchronize subsequent uplink transmissions from the UE, an initial uplink resource grant for a subsequent transmission, and an assignment of a temporary Cell Radio Network Temporary Identifier (C-RNTI). Once the Random Access Preamble is transmitted and regardless of the possible occurrence of a measurement gap, the UE monitors the PDCCH for RA response identified by the RA-RNTI. The RA response window starts at the subframe that contains the end of the preamble transmission plus three subframes and has length RA-ResponseWindowSize subframes.
In a third step, the UE conveys a Layer 2/Layer 3 (L2/L3) message on the UL, which may be known as message3 or msg3. More specifically, the UE conveys the actual random access procedure message on the PUSCH, such as an RRC connection request, tracking area update, or scheduling request. The message is addressed to the temporary C-RNTI allocated in the RAR in the second step described above. The UE identity is also included in this message for use later by the eNB. If the UE is in the RRC connected state, the UE identity is the C-RNTI assigned to the UE, otherwise the UE identity is a core-network terminal identifier.
In a fourth step, a contention resolution message is transmitted on the DL, which may be known as message4 or msg4. The contention resolution message is addressed to the C-RNTI (if indicated in msg3) or to the temporary C-RNTI, and, in the latter case, echoes the UE identity contained in msg3. In case of a collision followed by successful decoding of msg3, the HARQ feedback is transmitted only by the UE which detects its own UE identity (or C-RNTI); other UEs understand there was a collision. After contention resolution, the C-RNTI may be used by the eNB to address the UE that successfully completed the initial random access.
In typical deployments of WLAN, carrier sense multiple access with collision avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed to perform a clear channel assessment (CCA), and a transmission is initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is deemed to be Idle. When the range of several APs using the same frequency overlap, this means that all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP which is within range can be detected. Effectively, this means that if several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be degraded. FIG. 5 illustrates a general illustration of the listen before talk (LBT) procedure.
Typically, the spectrum used by LTE is dedicated to LTE. This has the advantage that LTE system is not concerned about coexistence issues or uncertainties in channel access. As a result, the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited and is not always able to meet the ever-increasing demand for larger throughput from applications/services. Therefore, a new industry forum has been initiated to extend LTE to operate entirely on the unlicensed spectrum in a standalone mode, which is referred to as “MuLTEfire” in marketing terms by certain sources. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LTE needs to consider the impact of LBT on UL procedures such as random access.
The existing RA procedure in LTE assumes complete subframes can always be transmitted and does not take into account the LBT process. Currently, the RA response from the eNB is scheduled using only the PDCCH, which will not be available if transmission is not feasible in the first three symbols of a DL subframe due to LBT. The channel access opportunities for standalone LTE-U may be very limited and transmission or reception of complete subframes may not be possible when LBT is a requirement.